“Engineering Biological Cell Access”

Nicholas Melosh, Ph.D

101 Goergen

The cell membrane is one of the most vital components of a cell; the gate-keeper into and out of the cytoplasm. Studies ranging from neural activity to drug discovery to fundamental cell physiology rely upon controlling electronic or chemical flow across this barrier. Unfortunately, our ability to artificially control cellular access is surprisingly poor; current techniques often involve creating holes in or puncturing cell membranes that often cause rapid cell death. Yet precise, non-destructive access to the cell interior can lead to an impressive range of new techniques and devices, ranging from neural-prosthetic interfaces, electrical cell monitoring for disease, to rapid drug screening. These types of technologies are particularly important for the recently announced “Brain Activity Map” and for harnessing the potential of induced pluripotent stem cells as disease models and clinical indicators.

One of the critical problems is that we have little knowledge of what happens when the lipid membrane interacts with cell-penetrating materials. Here we explore how inorganic materials with nanoscale molecular functionalization influence and control this vital interface. We discovered that by selectively tuning the hydrophobicity and flexibility of the molecular functionalization we can replicate many of the design features of natural transmembrane proteins, providing non-disruptive mechanical, electrical and chemical access into the cell. These “Stealth” probes spontaneously insert into lipid membrane, and form tight lipid-post junctions. Force testing with atomic force microscopy (AFM) provides a quantitative measure of interfacial adhesion, which surprisingly reveals significant adhesion energy differences between molecular groups with similar hydrophobicity but different mobility. This contradicts traditional ‘hydrophobicity indices’ as indicators of molecular stability in lipid membranes.

Similarly, we achieved high-efficiency chemical delivery and control by mimicking nature gap junction proteins to create ‘nanostraw’ arrays. These nanoscale (100-500 nm) diameter straws are formed based on simple water filtration membranes, and spontaneously penetrate cells plated onto them. We find these provide permanent conduits into cells, permitting delivery of a wide variety of materials that could normally not pass through the cell wall. These two technologies are examples of how the combination of physical sciences and biology can create new beneficial devices that would not be possible in isolation.